At Hereford Heights in Marton, the datum offset between the old Wellington Vertical Datum 1953 and the current New Zealand Vertical Datum 2016 was 0.543 metres. That might sound like a surveying footnote, but when your stormwater outfall design, finished floor levels, and flood hazard assessment are all based on elevation, a 543 mm error propagates through every calculation. This post explains the datum transition, how to identify which datum your LiDAR and survey data are in, and how to apply the correct offset.
What Are Vertical Datums and Why Do They Change?
A vertical datum is the reference surface against which all elevations (reduced levels) are measured. In New Zealand, the historical approach was to define local datums based on tide gauge observations at specific ports. Wellington Vertical Datum 1953 (WVD1953) was based on mean sea level observations at the Wellington tide gauge, and for decades it served as the de facto standard for engineering and surveying work across the lower North Island.
The problem is that WVD1953 is not a uniform surface. Because it was derived from a single tide gauge and propagated via levelling networks, the relationship between WVD1953 and true mean sea level varies with distance from Wellington. Tectonic movement, post-glacial rebound, and the limitations of the original levelling runs mean that WVD1953 elevations in Hawke's Bay, Manawatu, or Auckland do not relate to mean sea level in the same way they do in Wellington.
In 2016, Land Information New Zealand (LINZ) introduced the New Zealand Vertical Datum 2016 (NZVD2016). This datum is based on a gravimetric quasigeoid model (NZGeoid2016) and provides a nationally consistent reference surface that closely approximates mean sea level across the entire country. NZVD2016 is now the standard datum for all new survey data, LiDAR capture, and engineering design in New Zealand.
How Big Is the Offset?
The offset between NZVD2016 and WVD1953 is not a single number. It varies by location because WVD1953 is not a uniform surface. LINZ publishes the offsets through the NZVD2016 vertical datum relationship grids, which provide the conversion value for any given coordinate.
Some representative offsets across regions where SAE operates:
- Marton (Rangitikei): approximately +0.54 m (NZVD2016 is higher than WVD1953)
- Napier (Hawke's Bay): approximately +0.29 m
- Auckland (central): approximately +0.34 m
- Hamilton: approximately +0.37 m
At Hereford Heights in Marton, the offset was +0.543 m. This means a point surveyed at RL 100.000 m in WVD1953 would read RL 100.543 m in NZVD2016. If you mix data from both datums without applying the offset, every elevation in your design is wrong by more than half a metre.
Where Datum Errors Cause Real Problems
A 500 mm elevation error is not abstract. It has direct consequences across multiple engineering disciplines within a subdivision project.
Finished floor levels. Council consent conditions typically specify minimum finished floor levels (FFLs) relative to the 1% AEP (1:100-year) flood level plus freeboard. If the flood level is defined in NZVD2016 but your site survey is in WVD1953, and you set your FFL based on the survey without converting, every house will be built approximately 500 mm lower than intended. In a flood event, that 500 mm is the difference between dry floors and water through the living room.
Stormwater outfall design. Gravity stormwater systems rely on a continuous fall from the upstream collection point to the outfall. If the outfall invert level is taken from LiDAR in NZVD2016 but the upstream network is designed from a topographic survey in WVD1953, the calculated fall is wrong. A 500 mm error could turn a viable gravity system into one that requires pumping, or conversely, could mask an existing fall constraint.
Flood hazard assessment. Regional council flood maps are increasingly produced from models built on NZVD2016 LiDAR. If you compare those flood levels against a site survey in WVD1953, you will either overestimate or underestimate the flood risk by the datum offset. For a site near the margin of a flood hazard overlay, this error determines whether the site is classified as flood-prone or flood-free.
Wastewater connections. Connecting a new gravity sewer to an existing council main requires accurate invert levels on both the new and existing infrastructure. If the existing as-built records are in WVD1953 (common for older networks) and your design survey is in NZVD2016, the connection detail will be wrong unless you convert one dataset to match the other.
How to Identify Which Datum Your Data Uses
The first step in any project is to confirm the datum of every dataset you are working with. The key sources to check are:
LiDAR data. Most LiDAR captured after 2016 (including the national LiDAR programme managed by LINZ) is in NZVD2016. The metadata file accompanying the dataset will specify the vertical datum. If the metadata is missing or ambiguous, check the LINZ Data Service entry for that capture.
Topographic survey. The survey report or title block on the survey plan should state the vertical datum. If it references "NZVD2016" or "NZGeoid2016," the data is in the current datum. If it references "WVD1953," "Moturiki," "Lyttelton," or another local datum, conversion is required.
Council flood maps and design levels. Check the technical report or metadata for the flood study. Older HBRC and Horizons flood studies may use WVD1953 or local datums. Newer studies are typically NZVD2016.
Existing as-built records. As-built plans for older council infrastructure are almost always in the local datum that was current at the time of construction. For Rangitikei District, that is typically WVD1953.
Applying the Offset
Once you have identified the datums of your datasets, conversion is straightforward. LINZ provides the official conversion grids, and most survey software (including Trimble Business Centre and Leica Infinity) can apply the conversion automatically. For manual conversion in Civil 3D or similar design software, the offset is applied as a uniform vertical shift across the site, which is sufficiently accurate for sites smaller than a few kilometres across.
The critical discipline is documentation. Every engineering drawing, design report, and calculation set should state the vertical datum used. At Hereford Heights, every drawing title block included "Vertical Datum: NZVD2016" and the design report stated the offset value and the source of the conversion. This removes any ambiguity for the council reviewer, the surveyor, and the contractor.
The transition from WVD1953 to NZVD2016 introduced datum offsets of up to 500 mm or more across New Zealand. Every dataset in a subdivision project, including LiDAR, topographic surveys, flood maps, and as-built records, must be checked for its vertical datum before use. A 500 mm error in elevation propagates directly into finished floor levels, stormwater gradients, and flood hazard assessments. Confirm the datum. Apply the offset. Document it on every drawing.